Real confusion about when to delete QNetworkReply object

[jeremy_k]

For Qt 5.14 and later, theres QNetworkAccessmanager::setAutoDeleteReplies() which calls deleteLater() after emitting the finished signal.

[hbatalha]

@JoeCFD Thanks I understand now.
I didn't know that it could finish immediately and the slot may not be called. What would happen if that happens. I am curious.

/* check reply is null or not first */

Why would we do something like that. Is there any chance that the QNetworkAccessManager::get() may return a nullptr?

[hbatalha]

@fcarney said in Real confusion about when to delete QNetworkReply object:

connect(reply, &QNetworkReply::finished, reply, &QObject::deleteLater);

So this is the same as @jeremy_k answer: QNetworkAccessmanager::setAutoDeleteReplies() ?

[hbatalha]

@jeremy_k So that will eradicate the need to call deleteLater?

[JKSH]

@hbatalha said in Real confusion about when to delete QNetworkReply object:

So that will eradicate the need to call deleteLater?

Yes, that means you don't need to call deleteLater() yourself. But you must still set your pointer to nullptr.

OR, avoid storing the QNetworkReply as a member variable.

[jeremy_k]

@JoeCFD said in Real confusion about when to delete QNetworkReply object:

Normally, you may do the following because it may finish immediately and the slot may not be called. Sigh!

Can you provide a citation? QNetworkAccessManager creates its own worker thread(s), and seems to use queued connections for communication with the worker objects that carry out the requests. I also don't see evidence of this extra check in the HTTP example

A demonstration that works for me (but proves nothing):

#include <QCoreApplication>
#include <QNetworkAccessManager>
#include <QNetworkReply>
#include <QUrl>
#include <QNetworkRequest>
#include <QDebug>
#include <QThread>

int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);
    QNetworkAccessManager manager;
    QUrl url("http://forum.qt.io");
    QNetworkRequest request(url);
    qDebug() << "Fetching" << url;
    QNetworkReply *reply = manager.get(request);
    QThread::sleep(60); // Wait a minute!
    QObject::connect(reply, &QNetworkReply::finished, []() {
        qDebug() << "request finished";
        QCoreApplication::instance()->quit();
    });
    return a.exec();
}

[hbatalha]

@JKSH

Yes, that means you don't need to call deleteLater() yourself. But you must still set your pointer to nullptr.

I only need to call it once in the constructor?

OR, avoid storing the QNetworkReply as a member variable.

The only reason I have it as a member variable is because I need a way to call abort() to cancel a download.

[JoeCFD]

@hbatalha said in Real confusion about when to delete QNetworkReply object:

@JoeCFD Thanks I understand now.
I didn't know that it could finish immediately and the slot may not be called. What would happen if that happens. I am curious.

Read my code: if it finishes, you call the slot directly.

/* check reply is null or not first */

Why would we do something like that. Is there any chance that the QNetworkAccessManager::get() may return a nullptr?
Pure habit.

[JoeCFD]

@jeremy_k examples often show how things work. However, it may not include all you may need. I use it in posting and experienced issues.

[JKSH]

@hbatalha said in Real confusion about when to delete QNetworkReply object:

@JKSH

Yes, that means you don't need to call deleteLater() yourself. But you must still set your pointer to nullptr.

I only need to call it once in the constructor?

You must set your pointer to nullptr every time the object is deleted.

[hbatalha]

@JKSH This is how I did it

QObject::connect(&net, &QNetworkAccessManager::finished, [this, dest_file, url, fileDownloadRequestType](QNetworkReply* reply)
    {
        finished = true;

        if(! reply->error())
        {
            QByteArray data = reply->readAll();

            QFile file(dest_file);

            if( ! file.open(QIODevice::ReadWrite | QIODevice::Truncate))
                Util::displayErrorMessage(tr("Something went wrong"), tr("Could not save the file"), this, true);

            file.write(data);
            file.close();

            ui->msg_label->setText(tr("Download Complete"));
            ui->progress_label->setText("");

            exe = dest_file;

            if(fileDownloadRequestType == FileDownloadRequestType::UPDATE)
                handleUpdateDownloadFinished();
        }
        else if(reply->errorString() != "Operation canceled")
        {
            LOG_ERROR("Error Downloading from " + url + "\nDescription: " +  reply->errorString());
            reply = nullptr;
            Util::displayErrorMessage(tr("Network Error"), tr("Please try again later"), this, true);
        }

        m_reply = nullptr;
    });

[jsulm]

@hbatalha But now you don't call deleteLater() on reply, right?
You need to call deleteLater() AND set the pointer to nullptr afterwards.

[hbatalha]

@jsulm said in Real confusion about when to delete QNetworkReply object:

@hbatalha But now you don't call deleteLater() on reply, right?
You need to call deleteLater() AND set the pointer to nullptr afterwards.

I didn't call it because I called QNetworkAccessmanager::setAutoDeleteReplies() in the constructor.

[hbatalha]

Update:
So after running some tests and downloading multiple files I noticed a strange thing:
The slot connected to &QNetworkAccessManager::finished gets called more than once if I give it some time and keeps getting called. And because I set the reply to null in the first call, in the second call it overwrites the just downloaded file and writes null, making the FileDownloader pretty much useless.

After some debugging I found out that because FileDownloader::download everyt ime is called with the same object in the MainWindow it the QNetworkAccessManager object gets connected more than once and thus calling the slot more than once.

I have a few options to solve this:

• Check if ( nullptr != m_reply ) before connecting like @JoeCFD suggested in his answer
• Disconnecting the QNetworkAccessManager object and connecting it again
• Do not use lambda and create a method for that and connect it once in the constructor ( this is the option IMO but because I use some variables that come from the FileDownloader::download parameters inside the slot, it makes it difficult to use this option unless I created the member variables for those parameters.

[SGaist]

If you have some stuff that is associated to the reply, you can use dynamic properties to keep everything in one place or use a QMap to store the reply specific data that you will then grab in your lambda.

[hbatalha]

@SGaist

you can use dynamic properties to keep everything in one place

Didn't understand what you meant

use a QMap to store the reply specific data that you will then grab in your lambda

Could you provide a minimal sample code on how to do that